In recent years, secondary batteries have been used in various products. The secondary battery includes a battery element in which positive electrodes, separators and negative electrodes are stacked. It is important that, in the battery element, the positive electrodes and the negative electrodes are stacked with the separators interposed therebetween without being displaced. A displacement in stacking causes deterioration in battery performance or battery life.
To avoid such displacement of the positive electrode and the negative electrode, there has been proposed a technique in which a packaged positive electrode and a negative electrode are stacked, the packaged positive electrode being obtained by previously forming two separators in a bag shape and disposing a positive electrode in the bag, thus realizing quick and accurate stacking of the positive and negative electrodes (see Patent Literature 1). According to Patent Literature 1, positioning and stacking of the negative electrode and the separators also enables accurate positioning between the negative electrode and the positive electrode inside the separators. In this technique, the separators and the negative electrode are formed to have approximately the same size, and stacking thereof while aligning outlines thereof enables alignment between the negative electrode and the positive electrode inside the separators.